A protein of invertebrate origin, isolated from the coelomocytes of the sea star Asterias forbesi, has been shown to have remarkable effects on mammalian thymus-derived lymphocytes (T cells). These include suppression of cell division of these cells stimulated in vivo by thymus-dependent alloantigens and similar suppression of T cells stimulated by lectins such as concanavalin A and phytohemagglutinin, or by alloantigens in the mixed lymphocyte response in vitro. Interestingly, lower concentrations of this extract of sea star coelomocytes result in stimulation of T cells in lectin-induced T-cell division and appear to act as an agent which enhances T-cell division under those circumstances. The proposed research detailed in this application is principally concerned with examination of the mechanisms of cellular immunity. We have recently shown a O-cell requirement for an immunocompetent cell- mediated cytolysis using LCM virus in vitro and in vivo. LCM-infected L cells will be used to demonstrate the available antigens triggering this mechanism. The effect of antibody in blocking specific cytolyis will be investigated.